British photographer John Bradley, Ivan Rassimov, is dispatched to photograph wildlife deep in the Thai rainforest. In Bangkok, he attends a boxing match with a woman who grows frustrated and walks out on him; a mysterious man tailed him to a bar, confronts him with a knife, and is met with a swift counterattack from John, who kills the assailant and then flees into the night.
The following day, John rents a canoe and hires a guide to take him down the river deeper into the jungle. The guide, Tuan Tevan, voices worries about traveling so far, but John is steadfast, intending to press on for one more day before turning back.
John falls asleep and awakens to find his guide dead. A native tribe captures him in a net and carries him to their village, where the chief, Lahuna, Ong Ard Lahuna, is told they have captured a large fish-man. John is hung in a net from a pole and witnesses the execution of two war criminals by the tribes at war with a neighboring, more primitive cannibal clan, the Kuru. John labels his captors as murderers, even as he struggles to understand their world.
Hung at length, John catches the eye of Marayå, the chief’s daughter, who convinces her father that John is not a fish-man but simply a man. Lahuna agrees to release John as Marayå’s slave and locks him in a shack, where Taima, Prasitsak Singhara, an English-speaking governess to the tribe’s child, explains that John will be freed when Marayå is soon married to Karen. Lahuna interrupts and unties John because it is the day of the Feast of the Sun. A helicopter passes overhead, and when John attempts to be rescued, he is subdued by warriors, though Marayå intervenes. John then plans an escape with Taima’s help.
One month later, a building accident kills a young laborer. As John attends the funeral ceremonies and watches the natives’ rites with growing awe, Taima tells him that now is his time to escape. He does, but Karen and a group of warriors corner him, and he kills Karen. After this, the tribe accepts John as one of their own, subjecting him to trials and torture until he is released and welcomed as a warrior. He uses his knowledge of modern medicine and technology to aid the tribe, but his growing influence makes him an enemy of the tribe’s Witch Doctor, Song Suanhud.
John and Marayå become fond of each other and soon marry. Their union is blessed with a pregnancy, but a black butterfly flies overhead, signaling doom over their romance and future.
Six months after his capture, John fights alongside the tribe to repel an attacking party of Kuru cannibals who are preying on a young girl. He participates in dangerous skirmishes he once condemned, and when Marayå’s pregnancy worsens and she goes blind, he decides to carry her back to civilization for modern medical care. Taima helps them escape, but she is punished, and John and Marayå are forced to return to the village.
Marayå goes into labor, and John rejects the Witch Doctor’s help. The Kuru return, setting fire to the village as John and the remaining warriors scramble to defend it. He whisks Marayå to safety, but when she reveals that a black butterfly still flies overhead, she foresees death. Marayå dies after childbirth, and John wanders the jungle in a reflective haze, recalling her memory. A final helicopter drifts above as he contemplates his fate and chooses to remain with his tribe, hoping to endure the world he has become part of.
